Advice on adding radiators to an extension



I would like to add two new radiators to a single storey rear extension I am building on my house. My boiler has enough spare btu's to cope with the extra rads. Is it possible to connect straight to the flow and return pipes from the radiator that serves the upstairs bedroom about one metre away from the boiler and then tee from this to the two new rads? The new rads are approximately five and seven metres from the boiler and all of the pipe work is fifteen mm. This method will save a lot of hassle if it is possible. I am confident with pipe fitting but would like some advice on the technical aspects please. Any help is greatly appreciated, thanks.
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You can feed 3 rads from 15mm flow and returns, yes, it would be better with 22mm but it will work from the 15's, no more than 3 Rads though
Thanks for the quick reply. And the advice. the pipes are going to be concealed and it would be bad to find out it doesn't work afterwards, rads won't be fitted for a while so I only get one shot at getting it right. Thanks again
You can feed 3 rads from 15mm flow and returns, yes, it would be better with 22mm but it will work from the 15's, no more than 3 Rads though

that isn't necessarly correct as it depends on the rad sizes, as a rule of thumb 15mm will do 15,000 btu's, however that is also effected by the lenght of pipe runs so it may be less
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Thanks cbf. It's no more than 9000 btu so that should be ok then.
it's not just the new rads you have to think about you also have to consider which rads are already being fed by the 15mm that you want to connect onto, i've seen loads of times 22mm going into the floor from the airing cupboard then straight away split 15mm to feed all upstairs & 15mm feeding all downstairs
Thanks again cbf. The system is exactly as you describe, a short length of 22mm with a manifold with 15mm outlets. There is only one rad being fed by the pipe I want to tee off, a 900x450. I am proposing to use 15mm jg plastic pipe run in conduit in the floor screed with the flow and return from the two new rads meeting up with the flow and return from the pipe I want to connect to via two tees part way up the downstairs wall. This is so I have no fittings in the floor screed itself. So the plan is to have, from the manifold, two 1m lengths of pipe,(flow and return) teed to the existing rad, which continues through the wall and down to the ground floor, then say about 450mm up the downstairs wall a tee on both the flow and return pipe which will connect to the two new rads, approx 3 and 5 metres away respectively. I know this may not be ideal but I have an aversion to exposed pipes and cables and to connect two new runs direct to the manifold is going to be a major headache due to the layout and existing pipes and cables that will have to be rerouted. Sorry if this reply is a bit long winded and thanks again for your advice.
You might be better off to do this as it will halve the resistance of the pipe run to the new rads and give you better performance with less balancing problems!

Use single lengths of 15 mm plastic tube to each of the new rads. That has a lower flow resistance than copper with joints.

Tee them seperately off your chosen supply 15 mm pipe.

That would be one of the few times that I would want to use plastic pipe!

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